1. Field of the Invention
This invention relates generally to an interrupting apparatus having improved interruption capability and more particularly to an interrupting apparatus having a series-connected combination of two or more interrupting units such as a vacuum interrupter and a non-vacuum interrupter such as a sulfur hexafloride SF.sub.6 gas-blast interrupter, an air blast interrupter, or an oil circuit interrupter.
2. Description of the Prior Art
At present in the power industry there is a significantly improved interruption capability with a great increase in both the rated voltage and the interrupting current of interrupters being utilized in alternating current systems; however, interrupters having still higher interrupting capabilities, that is, the capability of withstanding steep current change rates (di/dt) and steep voltage change rates (dv/dt) in the proximity to a current zero, are necessary.
On the other hand, due to the remarkable increases in power consumption, direct current transmission systems which are steadier and more economical have been put into practice and thus direct current interrupters of various types are being manufactured. Unlike alternating current interrupters, direct current interrupters require a means for establishing a current zero since direct current, as such, has no current zero. Therefore many possible methods have heretofore been considered; however, the most practical method available at the present is a system wherein a high frequency alternating current is superimposed on the direct current in order to forceably establish a current zero for successful current interruption. To apply direct current interrupters using this method to the extra high voltage class (EHV) or the ultra high voltage class (UHV) direct current transmission systems, such interrupters should be provided with high interrupting capabilities in order to withstand steep current change rates (di/dt) and steep voltage change rates (dv/dt) in the same manner as in alternating current transmission systems.
Hereinafter, a detailed description will be presented illustrating the operation of direct current interrupters employing the above discussed method for producing successful interruptions by superimposing a high frequency alternating current on the direct current in the direct current transmission system.
FIG. 1 is a schematic diagram illustrating the connection of a direct current interrupter to a direct current transmission line. In FIG. 1, it is assumed that an alternating current from an alternating current generating system (not shown) is converted into a direct current 3 by means of an alternating current to direct current converter 1. The converted direct current 3 is transmitted in the direction of the arrow through a smoothing reactor 2 connected in series with the line and through a direct current interrupter 4. The current interrupter 4 includes a vacuum interrupter 5 coupled in series with an SF.sub.6 gas-blast interrupter 6 through which the direct current 3 passes. Coupled across the vacuum interrupter 5 and the SF.sub.6 interrupter 6 is the parallel combination of a conventional high frequency current generator 7 (not shown in detail) and an energy absorber 9 which will be further described below.
Now assuming that an interruption of the circuit is required, the direct current interruption will be made in such a method that, first of all, the vacuum interrupter 5 and the SF.sub.6 gas-blast interrupter 6 are actuated to provide sufficient clearance between their electrodes. Following this opening operation, the high frequency current generator 7 is energized to produce a high frequency alternating current 8, which is then fed into the circuit represented by the broken line, and thus will be superimposed over the direct current 3 within the interrupters 5 and 6.
The high frequency current generator 7 comprises, for example, various switching devices and a capacitor coupled in series. A charging device which functions to charge the capacitor is connected thereacross. The charging current (the high frequency current 8) flows in a direction opposite that of the direct current 3. Thus, this superimposed current serves to establish a current zero within the interrupters 5 and 6, such that current interruption can be achieved. Alternatively, the high frequency current generator may be formed by coupling a capacitor (not shown) in series between the interrupters 5 and 6 to produce a high frequency current 8 by utilizing the negative resistance characteristics of the arc produced during the interruption.
At the instant of current interruption, the smoothing reactor 2 accumulates a great amount of energy which is determined by the values of the interrupted direct current 3 and the inductance of the smoothing reactor 2. This energy is absorbed by the energy absorber 9. The energy absorber 9 can be formed by a large capacity resistor or a resistor having non-linear characteristics such as, for example a varister which primarily consists of zinc oxide. The voltage limited by this energy absorber 9, that is, the voltage represented by reference numeral 10 in FIG. 2, will be given as a recovery voltage of the interrupters 5 and 6.
FIG. 2 illustrates the waveform of the above-described phenomena. As can be seen, the interrupters 5 and 6 should withstand the steep current change rate of the high frequency current 8 represented by the dotted line, the rate of voltage rise of the recovery voltage, and the high recovery voltage 10 limited by the energy absorber 9. Moreover, after the energy stored in the smoothing reactor 2 is discharged, the interrupters 5 and 6 must withstand the voltage which still remains as developed by the direct current converter 1.
The constants for the phenomena described above are determined by the values such as the interrupted current, the voltage of the circuit in which the interrupters are used, and the voltage limited by the energy absorber; however, the performance required for the interrupters and the technology available at present are inevitably restricted, and thus the existing alternating current interrupters are not sufficient in their capabilities. For example, the steep current change rate of the high frequency current 8 shown in FIG. 2 ranges from 50 to 150 A/.mu. sec or higher, the rate of rise of the recovery voltage during the initial period also ranges from 5 to 10 KV/.mu. sec or higher, and the recovery voltage, for instance in the case of a circuit having a voltage of 250 KV, reaches a maximum of about 420 KV to 440 KV.
When such severe duty cycles are compared with the interruption performance of vacuum and SF.sub.6 interrupters, which are considered to be the most superior interrupters available at present, it can be seen that the highest one among such vacuum interrupters would range from 150 A/.mu. sec up to 300 A/.mu. sec in the steep current change rate, and may withstand as high as 50 KV/.mu. sec in the rate of voltage rise. However, the ratings of vacuum interrupters being manufactured for use in the present alternating current systems range only from 72 KV to approximate 125 KV, and moreover the most significant disadvantage is that there is the danger of the occurrence of reignition since countermeasures to prevent such occurrence have not been perfected, and in the case of the direct current systems, if reignition has occurred, it is impossible to provide re-interruption.
The characteristics of typical SF.sub.6 gas-blast interrupters in the proximity of a current zero generally range from 20 A/.mu. sec to 30 A/.mu. sec, or at the highest up to 50 A/.mu. sec, and 8 KV/.mu. sec for the maximum dv/dt.
These values represent the highest possible interrupting capability for their actual duty cycles. Thus it is impossible to impose duty cycles on these interrupters in excess of the above described values. Obviously, then, there is a great difficulty in providing either type of interrupters for use in the direct current transmission circuitry with satisfactory performance. On the other hand, the interrupting capabilities required for interrupters in alternating current circuitry are considered to be at least in the range of 40 KA to 50 KA even for those systems having a rating of 500 KVA, and such needs are anticipated to significantly increase in the future. Moreover, due to the short-line fault interruption or pull-out interruption, higher performance interrupters are presently increasingly required. The highest values in performance under these conditions can be represented such that in the case of a 275 KV, 50 HZ system, for example, when the interruption current is in the range of 63 KA (r.m.s), the rate of current fall (di/dt) reaches as high as 30 A/.mu. sec, and the rate of voltage rise reaches up to 10 KV/.mu. sec. The conditions are considered to have already exceeded the limits of interruption capability of even SF.sub.6 gas-blast interrupters that are regarded as the most suitable interrupters for use in the EHV or UHV systems.
For such applications, a series connected combination of a vacuum interrupter and a non-vacuum interrupter, such as an SF.sub.6 gas-blast interrupter, has been utilized. This device, referred to as a hybrid-type interrupter, is desirable because it combines the high current interruption capability of a vacuum interrupter with the superior voltage withstanding capacity of an SF.sub.6 gas-blast interrupter. However, a mere series-connection of both types of interrupters cannot make effective use of their advantages. Therefore, hybrid-type interrupting devices are needed wherein, during the period from a current zero to the rise of the recovery voltage, vacuum interrupters serve to control the largest share of the recovery voltage and wherein SF.sub.6 gas-blast interrupters serve to control the largest share of the increased recovery voltage occurring thereafter.